Possible bugs on histogram projection

[wx2017]

I used the following macros to analyze the following file on ROOT 6.12 and also 6.16 on Ubuntu 18.04.2 LTS. I expected the histogram h and h2 are identical. However, the histogram h2 miss the peak at 1.0.

Thanks

–Wei

///////////////////////////////

The ROOT file is at:
http://www.physics.purdue.edu/~wxie/tmp/

The macro is the following

{
TFile f("Fig_9_0.root");
nt->Draw("abs(tw1-tw2)");
nt->Draw("abs(tw1-tw2)>>h(100, 0, 1)", "", "same");
h->SetLineColor(2);

nt->Draw("abs(tw1-tw2)>>h2(100, 0, 1)");
h2->SetLineColor(4);

h->Draw();

h2->Draw("same");

c1->SetLogy(1);
}

---

ROOT Version: Not Provided
Platform: Not Provided
Compiler: Not Provided

---

[Wile_E_Coyote]

@couet It seems that the drawing option “same” neglects the given binning of the histogram (there is no any warning in the TTree::Draw method description). So, regardless of what one says, in this case it always uses (100, 0., 1.1).

[couet]

I have modified a bit your macro to make it more clear.

{
   TFile f("Fig_9_0.root");
   auto C = new TCanvas("C","C",600,800);
   C->Divide(1,2);

   C->cd(1);
   nt->Draw("abs(tw1-tw2)");
   nt->Draw("abs(tw1-tw2)>>h(100, 0, 1)", "", "same");
   printf("Number of bins in h = %d\n", h->GetNbinsX());
   printf("h xmin = %g\n", h->GetXaxis()->GetXmin());
   printf("h xmax = %g\n", h->GetXaxis()->GetXmax());
   h->SetLineColor(2);
   gPad->SetLogy(1);

   C->cd(2);
   nt->Draw("abs(tw1-tw2)>>h2(100, 0, 1)");
   printf("\nNumber of bins in h2 = %d\n", h2->GetNbinsX());
   printf("h2 xmin = %g\n", h2->GetXaxis()->GetXmin());
   printf("h2 xmax = %g\n", h2->GetXaxis()->GetXmax());
   h->SetLineColor(2);
   h->Draw("same");
   h2->SetLineColor(4);
   gPad->SetLogy(1);
}

it gives the output:

Number of bins in h = 100
h xmin = 0
h xmax = 1.1

Number of bins in h2 = 100
h2 xmin = 0
h2 xmax = 1

and the picture:

So it is clear that h and h2 have different upper limits and that option “same” for h forces the upper limit to be equal to the upper limit of the plot already on the canvas. I would suggest you do but the following way:

{
   TFile f("Fig_9_0.root");
   auto C = new TCanvas("C","C",600,800);
   C->Divide(1,2);

   C->cd(1);
   auto h = new TH1D("h","h",100, 0, 1);
   nt->Draw("abs(tw1-tw2)");
   nt->Draw("abs(tw1-tw2)>>h", "", "same");
   printf("Number of bins in h = %d\n", h->GetNbinsX());
   printf("h xmin = %g\n", h->GetXaxis()->GetXmin());
   printf("h xmax = %g\n", h->GetXaxis()->GetXmax());
   h->SetLineColor(2);
   gPad->SetLogy(1);

   C->cd(2);
   auto h2 = new TH1D("h2","h2",100, 0, 1);
   nt->Draw("abs(tw1-tw2)>>h2");
   printf("\nNumber of bins in h2 = %d\n", h2->GetNbinsX());
   printf("h2 xmin = %g\n", h2->GetXaxis()->GetXmin());
   printf("h2 xmax = %g\n", h2->GetXaxis()->GetXmax());
   h->SetLineColor(2);
   h->Draw("same");
   h2->SetLineColor(4);
   gPad->SetLogy(1);
}

The output is:

Number of bins in h = 100
h xmin = 0
h xmax = 1

Number of bins in h2 = 100
h2 xmin = 0
h2 xmax = 1